In recent years, a sprung weight of a vehicle tends to increase, wherein the sprung weight is on an upper side of a suspension device of the vehicle, for some reasons, wherein the reasons include a reason that there is a demand for low fuel consumption so that a fuel saving tire with a small rolling resistance is employed, and a fuel efficiency improving device is added, and a battery of a large capacity is required accordingly. Adoption of a fuel-saving tire leads to a decrease in friction coefficient between tire and road surface, whereas increase in sprung weight leads to an increase in suspension stroke.
Both of decrease in road surface friction coefficient and increase in suspension stroke tend to make yawing behavior of a vehicle have a nonlinear characteristic when steering operation is performed to steer steerable wheels, wherein the yawing behavior is around a vertical axis passing through the center of mass of the vehicle. Especially for an electric vehicle which is powered only by an electric motor as a power source, the nonlinear characteristic of yawing behavior is significant, because such an electric vehicle is provided with a large and heavy battery at a central place under a floor of a vehicle body. Moreover, increase in suspension stroke causes an increase in rolling motion of the vehicle that is a behavior of inclination around a longitudinal axis of the vehicle body.
For improvement of yawing behavior having a nonlinear characteristic, it is conceivable that suspension rigidity is enhanced by use of a high rigidity elastic bush or high rigidity insulator at a part to which the suspension device is attached, according to a technique as described in a patent document 1. On the other hand, for suppression of vehicle body rolling, it is conceivable that oscillation damping performance of a shock absorber of the suspension device is enhanced, for example, by use of a technique as described in a patent document 2.
However, the countermeasure of enhancing the suspension rigidity or the countermeasure of increasing the damping performance of the shock absorber causes an increase of spring coefficient of the suspension device, and thereby causes a new problem about oscillation and noise. Moreover, the countermeasure of increasing the damping performance of the shock absorber requires to provide the shock absorber with an additional damping force adjusting mechanism, and to add an actuator and others for actuating the damping force adjusting mechanism, and thereby causes a demerit of cost-up.